Linear motors are used in various types of systems, such as for positioning and moving applications, including machining and gantry type systems. The high performance systems often require moving elements subjected to high acceleration levels. In order to achieve such high acceleration, the linear motor must exert large forces upon the elements to be moved.
There are various configurations of linear motors, including flat motors, U-channel motors and tubular shaped motors. Different types of linear motors are also available, including brush, AC brushless, stepper, and induction motors. Common to most linear motors is a moving assembly, usually called a forcer or stage, which moves relative to a stationary platen (or path) according to magnetic fields generated by application of current through one or more associated windings. The windings can be on the forcer or at the platen depending on the type of motor. For example, in a permanent magnet linear motor, a series of armature windings can be mounted within a forcer that is movable relative to a stationary path. The path can include an array of permanent magnets configured to interact with the coils in the stage when energized with an excitation current.
Alternatively, in another type of conventional linear motor, permanent magnets can be part of a moveable stage with the coils situated in the platen. Usually, the permanent magnets are attached to a back iron plate above the coils, which are oriented along a path of travel. The magnets usually are rectangular in shape. The magnets are arranged along the back iron so that adjacent pairs of magnets have opposite magnetic pole orientations. The magnets can be oriented generally normal to the direction of travel or inclined at a slight angle from normal to an axis of the direction of travel for the linear motor. The inclined angle creates a flux distribution along the axis of movement, which is generally sinusoidal in nature. Such a resulting distribution due to the optimized motor geometry tends to reduce cogging during operation of the linear motor, which would otherwise occur if the magnets were aligned, normal to the axis of movement.
Although an inclined angle of the magnets can reduce some cogging, it presents a disadvantage in that a larger area typically must be covered by the rectangular magnets in order to sufficiently cover and interact with the coils of the armature. When the magnets are implemented with a larger area so as to reduce cogging effects, a larger footprint for the back iron also is required. This tends to increase the overall weight and size of the stage. Such increases in size and weight can present additional obstacles, such as in applications where there are size constraints and low mass is desirable. For example, as the mass of the stage increases, the available acceleration experiences a corresponding reduction, and the ability to stop the motor accurately also reduces because of the increased power dissipation needed to stop the motor.
As the use of linear motors in manufacturing equipment continues to increase, nominal increases in the speed of operation translate into significant savings in the cost of production. Accordingly, it is desirable to provide a magnet assembly that can be part of a high performance linear motor.